1) Field of the Invention
The present invention relates to an electrophotographic lithograph printing plate material.
More particularly, the present invention relates to an electrophotographic lithograph printing plate material having an enhanced sensitivity to semiconductor laser rays.
2) Description of the Related Arts
Generally, a conventional electrophotographic lithograph printing plate material has a photosensitive electrophotographic layer wherein electroconductive zinc oxide particles are dispersed as a photoconductive material. This type of lithograph printing plate material (known as a zinc oxide offset master material) is widely employed in the light printing industry, because it is cheap, and because the process for making a printing plate from the material is simple and easy.
In a conventional process for producing a lithograph printing plate from the above-mentioned printing plate material, a visible light-irradiation source, for example, a halogen lamp, is used. In this process, the visible light is irradiated to and reflected on an original image or picture and the reflected rays are irradiated to the photosensitive surface of the printing plate material. This method is referred to as a camera system printing plate-making method.
Due to the recent development of various recording machines and the spread of data digitalization a computer-to-plate type printing plate-making method is now widely used for the electrophotographic material. In this method, laser rays which can be controlled in accordance with computer data are applied to the photosensitive printing plate material surface as a scanning exposure.
Among the laser rays, semiconductor laser rays, which can be generated in a small size device and can be directly modulated, are most useful.
The zinc oxide offset master usable for the semiconductor laser rays is made from a lithograph printing plate material having a photosensitive electrophotographic layer spectrosensitized by a sensitizing dye and having an enhanced sensitivity at a wave length of 700 to 1000 nm, particularly 780 nm, of the semiconductor laser rays.
The sensitizing dye usable for the above-mentioned use is selected from polymethine type cyanine dyes. These polymethine type cyanine dyes are classified into two groups in accordance with the degree of capability thereof of being adsorbed by zinc oxide particles, i.e., low adsorption dyes which are adsorbed by the zinc oxide particles at a low adsorption rate, and high adsorption dyes which are adsorbed by the zinc oxide particles at a high adsorption rate.
The first group of low adsorption dyes, which will be referred to as low adsorption sensitizing dyes hereafter, includes the compounds, for example, of the formulae (I) and (II): ##STR1## most of which compounds have alkyl or alkylether radicals attached to the N atoms.
The second group of high adsorption dyes, which will be referred to as high adsorption sensitizing dyes hereafter, include the compounds, for example, of the formulae (III) and (IV): ##STR2## most of which compounds have acid radicals, for example, alkylsulfonic or alkylcarboxylic acid radicals, attached to the N atoms.
These two groups of sensitizing dyes have mutually inconsistent properties. Namely, the high adsorption sensitizing dyes are advantageous in that the resultant lithograph printing plate material exhibits an excellent heat resistance, but are disadvantageous in that the dark decay of the resultant lithograph printing plate material is undesirably increased. In comparison, the low adsorption sensitizing dyes are advantageous in that the resultant lithograph printing plate material exhibits a low dark decay, but are disadvantageous in that the heat-resistance of the resultant lithograph printing plate material is poor.
The term "heat-resistance" as used herein refers to a property such that, even when exposed to an action of heat, the photosensitivity of the lithograph printing plate material is not affected. The higher the heat resistance, the higher the durability in use and during storage and transportation. Therefore, the heat resistance is a very important property of the lithograph printing plate material.
When the dark decay is high, the potential of the surface of the printing plate material is lowered during the period between the charging step and the developing step, and thus the color density (darkness) of the resultant images on the printing plate material surface is reduced.
Many attempts have been made to simultaneously obtain both a high heat resistance and a small dark decay, but these attempts were not always successful in obtaining an electrophotographic lithograph printing plate material having both an enhanced heat resistance and a reduced dark decay.